Incineration of municipal solid wastes (MSW), biomedical wastes (BMW), and the like, produce combustion products containing various toxic metals. These metals and their compounds are known to be inimical to human health. Most of the toxic metals oxidize and condense to particulate form on cooling, and can therefore be filtered out by appropriate filter means. However, some toxic metals, particularly mercury and cadmium and their halogen and oxide compounds, tend to remain in vapor form on cooling from combustion temperatures. These metals and their compounds are not removable by filtration. On the other hand, these metals and compounds are toxic at the vapor phase concentrations that exist in the typical combustion gases and stack emissions. The environmental standards of several states now require a limit on mercury emissions of 50 micrograms per dry standard cubic meter (ug/DSCM), and 50 ug/DSCM for cadmium and thallium combined corrected to a reference concentration of oxygen or carbon dioxide content in the gas.
In the presence of hydrochloric acid (HCl) in the gas, mercury and cadmium compounds found in the waste combustion gases from MSW and BMW incineration are predominantly in the form of uncondensed vapor-phase chlorides. On cooling from the elevated combustion temperatures, both mercury and cadmium react as elements or as oxides with the HCl in the gas to form the vapor-phase chlorides.
It is known in the art that gas-phase mercury and cadmium chlorides may be removed from combustion gases by adsorption on high surface area sorbents, such as activated carbon, or by wet scrubbing with aqueous solutions. Wet scrubbing, which comprises treating the cooled combustion gases with an aqueous scrubbing solution in an efficient gas-liquid contactor, is conventionally employed to remove the acid gases, HCl and SO.sub.2, from the combustion gases. Wet scrubbers typically use an aqueous solution of alkali metal or alkaline earth carbonate or hydroxide for neutralization of the absorbed HCl. Although effective for HCl and SO.sub.2 removal, wet scrubbing has been found to be only partially effective for mercury and cadmium removal. This is chemically somewhat contradictory because both HgCl.sub.2 and CdCl.sub.2 are known to be very soluble in aqueous systems. The metal chlorides, HgCl.sub.2 and CdCl.sub.2, form dissolved chloride complexes in solution which have extremely low vapor pressures, and aqueous wet scrubbing would theoretically be expected to be very efficient for mercury/cadmium compound removal. However, this has not proved to be true in application.
The reasons for the failure of wet scrubbing when applied to removal of mercury compounds from waste incinerator combustion exhaust gases have been pointed out in U. S. Pat. No. 5,009,871 to Higuchi et al. The mercury compounds in solution were found to be highly susceptible to reduction to the elemental metal phase by reducing agents formed in situ in the absorbing liquor. The vapor pressure of the elemental metals is very high compared to the vapor pressure of the dissolved chloride compounds, and the slightest degree of reduction in the liquor system will generate mercury or cadmium as a separate metal phase and drastically increase their stack emission levels.
As is typically the case when absorbing residual HCl from the gas, the pH in a combustion gas wet scrubber is maintained above pH=7 by alkali injection. Higuchi et al. found that SO.sub.2 in the gas was absorbed in their caustic scrubbing solution to form alkaline sulfites and bisulfites. These are excellent reducing agents, and served to reduce the absorbed HgCl.sub.2 to elemental mercury.
Higuchi et al. determined the net absorption of mercury compounds in the gas to be a function of the chemical oxygen demand (COD, or reducing ability) of the caustic solution. At high COD levels (150 milligrams/liter), the net removal of mercury from the gas was found to be less than 10 percent. Higuchi et al.'s solution to this problem was to add compensating oxidizers, such as sodium hypochlorite, to the solution to prevent reduction of the dissolved mercury compounds. This is an expensive expedient. The COD balance is difficult to control by such means and the hypochlorite is an undesirable and corrosive contaminant. Even with adjusted solution COD (non-reducing solutions) the degree of mercury removal reported by Higuchi et al. for wet scrubbing yielded mercury emissions that were far in excess of the 50 ug/DSCM standard.
Removal of mercury and cadmium chlorides from combustion gases using activated carbon adsorption is also known in the art. U.S. Pat. No. 4,889,698 to Moller et al. discloses activated carbon addition in, upstream or downstream of, a spray drier scrubber for mercury and dioxin removal. Use of a powdered activated carbon as a supplement to spray drier technology was found to improve the removal of chlorodibenzo-dioxins and chlorodibenzofurans (hereinafter referred to as dioxins and furans) and mercury. Moller et al. found that efficient removal of the pollutants with powdered activated carbon adsorbent in spray drier scrubbing occurs when sufficient water is evaporated to cool the flue gas to 110.degree. C.-130.degree. C.
The normal gas cooling means in both MSW and BMW incineration is a waste heat boiler, and a conventional waste heat boiler cools the combustion gases to the 175.degree. to 250.degree. C. range, which is well above Moller's preferred lower sorption temperature levels. In a series of field tests, the U.S. Environmental Protection Agency (EPA) found mercury compound removal by activated carbon in the 160.degree. to 250.degree. C. range to yield residual gas-phase mercury levels that were highly excessive when compared to a 50 ug/DSCM emission limit. The results of an EPA test program of medical waste incinerator emissions, using activated carbon powder injection into the combustion gas, were reported in a paper by K. R. Durkee and J. A. Eddinger, entitled "Status of EPA Regulatory Program for Medical Waste Incinerators--Results of Emission Test Program", presented at the 11th Annual Incineration Conference, Albuquerque, N. Mex., May, 1992. Carbon injection downstream of waste heat boilers, followed by fabric filtration, gave exit gas mercury concentrations that were in the range of 284 to 587 ug/DSCM. Clearly, activated carbon treatment of the gas does not provide the required degree of removal to achieve compliance with a 50 ug/DSCM emission specification.
While both wet scrubbing and activated carbon systems have the capability for removal of toxic metal chlorides from the combustion gases, neither has the capability for removal of the elements or their oxides from the gas. Hall, Lindquist and Lungstrom, in an article entitled, "Mercury Chemistry in Simulated Flue Gases Related to Waste Incineration Conditions", in "Environmental Science and Technology", pp. 108-111, Volume 24 (1990) reported experiments on mercury removal with activated carbon. In the absence of HCl in the gas, only 13 to 20% of the mercury vapor was absorbed by the carbon over a temperature range of 160.degree. to 500.degree. C.
Further, the remaining vapor-phase mercury content was catalytically converted by the carbon to mercuric oxide. Therefore, in the absence of sufficient HCl in the gas to convert the mercury/cadmium metals and oxides to adsorbable chlorides, activated carbon adsorption does not provide significant removal of these toxic compounds. Additionally, because only the chloride form of these toxic metals is soluble in aqueous solutions, wet scrubbing does not remove elemental mercury, cadmium or their oxides from the gases.